hwdb: drop quotes from XKB_FIXED_*= properties

hwdb: drop quotes from XKB_FIXED_*= properties

The properties are not unquoted by udev, so the quotes effectively became part
of the value.

Even though those properties were added quite a while ago
(086c001e29a86287d7b639cb71d1fc6408920c53,
d7d31692bf7cde5dce7f4ed3cae429a5b302a9f0), they never started being used
(because of issues with having multiple layouts), see
https://gitlab.gnome.org/GNOME/mutter/-/issues/906,
https://bugzilla.gnome.org/show_bug.cgi?id=775681.

Let's remove the quotes while we still can.

From https://bugzilla.gnome.org/show_bug.cgi?id=775681#c7:
> Note to self: the values for XKB_FIXED_LAYOUT and XKB_FIXED_VARIANT are
> quoted, meaning that we need to remove the quotes before passing the values
> from udev_device_get_property_value() to xkb_keymap_new_from_names()
> otherwise the compilation of the keymap fails (please don't ask how I found
> out...)